"What is best?"
That's a bit like asking which baseball team is best... a guaranteed start to a "lively" discussion. We've had a few of those, and it would make sense to look at them - here are some examples:
More Tire Talk, Regulations say ST
tires n rims, 14 inch
Auto/Truck tires on 13 ft. Scamp?, Bias or radial?
One approach would be simply to get replacements which are the equivalent of the original equipment; if doing that, there are a couple of factors to consider...
The "load range
" is missing from the spec of the existing tires; that's the "B" or "C" which Tom mentioned. It is mostly an indication of the maximum operating pressure, which determines how much load can be carried. Modern passenger car and light
truck tires have switched to a "load index" system to identify capacity. Regardless of how it is stated, the capacity must be appropriate to the trailer (the total for two tires must be greater than the highest axle
load carried, and preferably greater than the axle weight
rating) and the pressure must be maintained high enough to deliver that capacity.
That "D" means "diagonal ply", which is more commonly known as "bias
ply", as Bill mentioned. It refers to the direction which the cords of the fabric in the tire structure run; it's a detail which doesn't matter to the customer, but I would be hesitant to believe any information from staff in a store or tire shop who is so ignorant of their own business that they don't know this. Only the smallest and least critical or lowest-speed applications have bias ply tires these days (meaning in this century) - the world uses radial
tires now, but I'm sure bias ply tires will work as well (or as poorly) as they did decades ago, or when Casita
saved a couple bucks by putting them on in 2000.
My 17' Boler
has P205/75R14 tires, which are a close size and capacity match to the original F78-14 tires. I need to replace them, and about the only thing certain is that I won't be using those sizes/types again.